New research could help explain why losing weight is so difficult — obesity may cause inflammation in the brain. And just a single high-fat meal could have the same effect.

NPR reports on a study in which researchers made rats obese by feeding them a high-fat diet. The rats developed inflammation in the hypothalamus, a part of the brain that helps control weight and hunger. What's more, the inflammation appeared after just a single day, before the rats actually gained any weight. To see if the effect appeared in humans, the researchers also did brain scans on 35 obese subjects, and found evidence consistent with inflammation in their hypothalami as well. The study authors write, "These findings collectively suggest that, in both humans and rodent models, obesity is associated with neuronal injury in a brain area crucial for body weight control."

It's probably not true that a single fatty meal will forever break your brain. Endocrinologist Rexford Ahima points out to NPR that the inflammation could actually be the brain's way of protecting itself from a fatty onslaught: "It may be a good thing," he says, that only turns bad if it's constant. Whatever the case, though, the recent study is more evidence that weight gain and loss are more complicated than calories in versus calories out. It's becoming increasingly clear that a number of physiological factors make it tough to lose weight — and these may include changes to the brain.